Shaped articles such as, for example, sheet, film, tubes, bottles, sleeves, and labels, are commonly used in various packaging applications. For example, film and sheet produced from polymers such as polyolefins, polystyrene, poly(vinyl chloride), polyesters and the like are used frequently for the manufacture of shrink labels for plastic beverage or food containers. It is desirable in many packaging applications that the shaped article exhibit properties such as, for example, good printability, high opacity, low density, low shrink force, good texture, recyclability, and high stiffness. For example, during recycling of the plastic material, labels are often separated from the rest of the container because of the presence of inks, glues, and other substances which can contaminant and discolor the recycled polymer. If the density of the label polymer is sustantially different from that of the container polymer, the separation of the label polymer may be carried out by a simple and economical flotation process in which the label polymer floats or sinks away from the other polymers. Unfortunately, the label and container, materials used in packaging often have similar densities that prevents the use of such flotation processes.
One approach for reducing the density is to introduce many small voids or holes into the shaped article. This process is called “voiding” and may also be referred to as “cavitating” or “microvoiding”. Voids are obtained by incorporating about 5 to about 50 weight % of small organic or inorganic particles or “inclusions” (referred in the art as “voiding” or “cavitation” agents) into a matrix polymer and orienting the polymer by stretching in at least one direction. During stretching, small cavities or voids are formed around the voiding agent. When voids are introduced into polymer films, the resulting voided film not only has a lower density than the non-voided film, but also becomes opaque and develops a a paper-like surface. This surface also has the advantage of increased printability; that is, the surface is capable of accepting many inks with a substantially greater capacity over a non-voided film. Typical examples of voided films are described in U.S. Pat. Nos. 3,426,754; 3,944,699; 4,138,459; 4,582,752; 4,632,869; 4,770,931; 5,176,954; 5,435,955; 5,843,578; 6,004,664; 6,287,680; 6,500,533; 6,720,085; U.S. Patent Application Publication No.'s 2001/0036545; 2003/0068453; 2003/0165671; 2003/0170427; Japan Patent Application No.'s 61–037827; 63–193822; 2004–181863; European Patent No. 0 581 970 B1, and European Patent Application No. 0 214 859 A2.
Although voided films are known, they frequently suffer from a number of shortcomings and often show inferior properties to the corresponding non-voided counterparts such as, for example, poor stiffness, insufficient opacity, high shrink force, and high surface roughness which make them less desirable for many packaging applications. For packaging labels, for example, it is often desirable for aesthetic purposes to have a high concentration of voids such that the voided film is opaque. Increasing the number of voids, however, can increase the surface roughness to the point that the printing quality, texture and feel, and seamability of the label are reduced. To address this problem, many voided films have multiple layers in which a non-voided surface layer is affixed to a void-containing core layer (by adhesion or coextrusion). The non-voided layer is applied because it provides a smoother surface than the voided layer. While this approach solves many of the above problems, production of such multilayer films is expensive and requires additional coextrusion or lamination equipment. Multilayered films also typically have a higher overall film density because of the lack of or decreased voiding on the surface and are not as desirable as monolayer films. It is also possible to introduce voids into containers such as, for example, a bottle or thermoformed tray. Voided containers are lightweight, require less polymer, and can be printed upon directly, thus eliminating the need for a label.
Conventional voiding agents suffer from several disadvantages. Inorganic agents like calcium carbonate, talc, silica, and the like may be used as voiding agents but, because inorganic substances are typically dense materials, the final density of the shaped article is often too high. In the case of voided films, for example, the reduction in density imparted by voiding is frequently offset by the weight of the inorganic agents.
Polyolefins such as, for example, polypropylene may be used as a voiding agents. Polyolefins, however, often do not disperse well and may require a compatibilizer such as, for example, a carboxylated polyethylene to obtain a uniform distribution of voids. When used with polyester polymers to produce voided films, polyolefins also tend to lower the polyester film surface tension and thereby reduce the printability of the film. Polyolefins are softer than the polyester at room temperature which sometimes lowers the overall film modulus to unacceptable levels. Finally, polyolefins are relatively inefficient voiding agents and large amounts are required to achieve the necessary density reduction. As discussed earlier, this leads to poor surface roughness and printing problems, thus making it difficult to use in single layer films.
Other polymeric voiding agents such as, for example, styrenics, polymethylpentene, polycarbonate, nylons, cellulosics, and the like, suffer from some of the same voiding efficiency problems as polyolefins. High modulus styrenics, like atactic polystyrene, are efficient voiding agents, but suffer from outgassing problems when mixed and processed at higher temperatures and, therefore, are useful only at low levels. Styrenics also tend to embrittle the film. Crosslinked styrene beads may be used to circumvent this problem, although these beads tend to be expensive. Cellulosics tend to be hygroscopic and require a separate drying and moisture removal step before incorporation into the polymer matrix. For voided shrink films, cellulosics also tend to produce undesirably high shrink forces.
There is a need, therefore, for a composition that would enable the production of voided, shaped articles such as, for example, film, sheet, bottles, tubes, fibers, and rods, having a lower density, good printability, lower shrink force, high opacity, and other desirable physical properties such as high stiffness and good texture and feel. In the case of films, there is a also need for a composition that would permit the preparation of single layer, void-containing films with acceptable printability, stiffness, and lower densities. Such a composition would have utility in the beverage and food packaging industry for the production of void-containing shrink labels.